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Chapter 16: Linking

16.1. References

16.1.1. Overview

On the Internet, resources are identified using URLs (Internationalized Resource Identifiers). For example, an SVG file called someDrawing.svg located at http://example.com might have the following URL:

http://example.com/someDrawing.svg

An URL can also address a particular element within an XML document by including an URL fragment identifier as part of the URL. An URL which includes an URL fragment identifier consists of an optional base URL, followed by a "#" character, followed by the URL fragment identifier. For example, the following URL can be used to specify the element whose ID is "Lamppost" within file someDrawing.svg:

http://example.com/someDrawing.svg#Lamppost

16.1.2. Definitions

URL reference
An URL reference is an Internationalized Resource Identifier, as defined in Internationalized Resource Identifiers [rfc3987]. See References and References and the ‘defs’ element.
URL reference with fragment identifier
An Internationalized Resource Identifier [rfc3987] that can include an <absoluteURL> or <relativeURL> and a identifier of the fragment in that resource. See References and the ‘defs’ element. URL reference with fragment identifiers are commonly used to reference paint servers.
external file reference
A URL reference or URL reference with fragment identifier which refers to a resource that is not part of the current document.
same-document URL reference
A URL reference with fragment identifier where the non-fragment part of the URL refers to the current document.
data URL
A URL reference to an embedded document specified using the "data" URL scheme [rfc2397]. Data URL references are neither external file references nor same-document URL references.
circular reference
URL references that directly or indirectly reference themselves are treated as invalid circular references. What constitutes a circular reference will depend on how the referenced resource is used, and may include a reference to an ancestor of the current element.
unresolved reference
A reference that is still being processed, and has not yet resulted in either an error or an identified resource.
invalid reference

Any of the following are invalid references:

  • A circular reference.
  • A URL reference that results in an error during processing.
  • A URL reference that cannot be resolved.
  • A URL references to elements which are inappropriate targets for the given reference shall be treated as invalid references (see Valid URL targets for appropriate targets). For example, the clip-path property can only refer to ‘clipPath’ elements. The property setting clip-path:url(#MyElement) is an invalid reference if the referenced element is not a ‘clipPath’.

Invalid references may or may not be an error (see Error processing), depending on whether the referencing property or attribute defines fallback behavior.

16.1.3. URLs and URIs

Internationalized Resource Identifiers (URLs) are a more generalized complement to Uniform Resource Identifiers (URIs). An URL is a sequence of characters from the Universal Character Set [UNICODE]. A URI is constructed from a much more restricted set of characters. All URIs are already conformant URLs. A mapping from URLs to URIs is defined by the URL specification, which means that URLs can be used instead of URIs in XML documents, to identify resources. URLs can be converted to URIs for resolution on a network, if the protocol does not support URLs directly.

Previous versions of SVG, following XLink, defined an URL reference type as a URI or as a sequence of characters which must result in an URL after a particular escaping procedure was applied. The escaping procedure was repeated in the XLink 1.0 specification [xlink], and in the W3C XML Schema Part 2: Datatypes specification [xmlschema-2]. This copying introduced the possibility of error and divergence, but was done because the URL specification was not yet standardized.

In this specification, the correct term URL is used for this "URI or sequence of characters plus an algorithm" and the escaping method, which turns URLs into URIs, is defined by reference to the URL specification [rfc3987], which has since become an IETF Proposed Standard. Other W3C specifications are expected to be revised over time to remove these duplicate descriptions of the escaping procedure and to refer to URL directly.

16.1.4. Syntactic forms: URL and <url>

In SVG, most structural relationships between two elements are specified using a URL value in an ‘href’ attribute.

To describe linking relationships, this specification uses two different data types in attribute and property values: URL and <url>. The linking guidelines in this chapter apply to URLs specified with either syntax.

The URL data type is a simple URL string. In SVG, most structural relationships between two elements are specified using a URL value in an ‘href’ attribute.

<url> is different from URL and represents a CSS url() function value. (See CSS Values and Units for further details [css-values]). <url>s may be used for presentation attributes and their corresponding CSS properties [css-values].

URL is not a valid value for presentation attributes for structural relationships purposes. No non-presentation attribute allows <url> as value.

SVG makes extensive use of URL references, both absolute and relative, to other objects. For example, a ‘linearGradient’ element may be based on another gradient element, so that only the differences between the two need to be specified, by referencing the source gradient with a URL in the ‘href’ attribute: 

<linearGradient id="SourceGradient">...</linearGradient>
<linearGradient id="MyGradient"
                href="#SourceGradient">...</linearGradient>

To fill a rectangle with that gradient, the value of the rectangle’s fill property may be set so as to include a URL reference to the relevant ‘linearGradient’ element; here is an example: 

<rect fill="url(#MyGradient)"/>

16.1.5. URL reference attributes

URL references are normally specified with an ‘href’ attribute. The value of this attribute forms a reference for the desired resource (or secondary resource, if there is a fragment identifier). The value of the ‘href’ attribute must be a URL.

Because it is impractical for any application to check that a value is an URL reference, this specification follows the lead of the URL Specification in this matter and imposes no such conformance testing requirement on SVG authoring tools. An invalid URL does not make an SVG document non-conforming. SVG user agents are only required to process URLs when needed, as specified in Processing of URL references.

16.1.6. Deprecated XLink URL reference attributes

In previous versions of SVG, the ‘href’ attribute was specified in the XLink namespace [xlink] namespace. This usage is now deprecated and instead URL references should be specified using the ‘href’ attribute without a namespace.

For backwards compatibility, the deprecated ‘xlink:href’ attribute is defined below along with the ‘xlink:title’ attribute which has also been deprecated.

Attribute definitions:

Name Value Initial value Animatable
xlink:href URL [URL] (none) (see below)

For backwards compatibility, elements with an ‘href’ attribute also recognize an ‘href’ attribute in the XLink namespace [xlink].

When the ‘href’ attribute is present in both the XLink namespace and without a namespace, the value of the attribute without a namespace shall be used. The attribute in the XLink namespace shall be ignored.

A conforming SVG generator must generate ‘href’ attributes without a namespace. However, it may also generate ‘href’ attributes in the XLink namespace to provide backwards compatibility.

This attribute is Animatable if and only if the corresponding ‘href’ attribute is defined to be animatable.

Name Value Initial value Animatable
xlink:title <anything> (none) no

Deprecated attribute to describe the meaning of a link or resource in a human-readable fashion. New content should use a ‘title’ child element rather than a ‘xlink:title’ attribute.

The use of this information is highly dependent on the type of processing being done. It may be used, for example, to make titles available to applications used by visually impaired users, or to create a table of links, or to present help text that appears when a user lets a mouse pointer hover over a starting resource.

The ‘title’ attribute, if used, must be in the XLink namespace. Refer to the XML Linking Language (XLink) [xlink].

When using the deprecated XLink attributes ‘xlink:href’ or ‘xlink:title’ an explicit XLink namespace declaration must be provided [xml-names], One simple way to provide such an XLink namespace declaration is to include an ‘xmlns’ attribute for the XLink namespace on the ‘svg’ element for content that uses XLink attributes. For example:

<svg xmlns:xlink="http://www.w3.org/1999/xlink" ...>
  <image xlink:href="foo.png" .../>
</svg>

16.1.7. Processing of URL references

URLs are processed to identify a resource at the time they are needed, as follows:

Legacy ‘xlink:href’ attributes are processed at the time a corresponding ‘href’ attribute would be processed, but only if no such ‘href’ attribute exists on the element.

Processing a URL involves three steps: generating the absolute URL; fetching the document (if required); identifying the target element (if required).

A URL reference is unresolved until processing either results in an invalid reference or in the identification of the target resource. Unresolved references in the non-presentation attributes of structurally external elements prevent the load event from firing. User agents may place time limits on the resolution of references that are not same-document URL references, after which the reference is treated as a network error (and therefore as an invalid reference).

For same-document URL references in a dynamic document, modifications or animations of attributes or properties, or removal of elements from the DOM, may cause an URL reference to return to the unresolved state. The user agent must once again attempt to resolve the URI to identify the referenced resource.

16.1.7.1. Generating the absolute URL

If the URL reference is relative, its absolute version must be computed before use. The absolute URL should be generated using one of the following methods:

The ‘xml:base’ attribute will only have an effect in XML documents; this includes SVG documents and XHTML documents but not HTML documents that are not XML. In contrast, a base element affects relative URLs in any SVG or HTML document, by altering the document base URL.

If the protocol, such as HTTP, does not support URLs directly, the URL must be converted to a URI by the user agent, as described in section 3.1 of the URL specification [rfc3987].

After generating the absolute URL:

As defined in CSS Values and Units, a fragment-only URL in a style property must be treated as a same-document URL reference, regardless of the file in which the property was declared.

16.1.7.2. Fetching the document

SVG properties and attributes may reference other documents. When processing such a URL, the user agent should fetch the referenced document as described in this section, except under the following conditions:

When fetching external resources from the Internet, user agents must use a potentially CORS-enabled request as defined in HTML [HTML] with the corsAttributeState as follows:

base

The request’s origin is computed using the same rules as HTML, with an SVG ‘script’ element treated like an HTML script element, and an SVG ‘image’ element treated like an HTML img element. The default origin behaviour must be set to taint.

A future SVG specification may enable CORS references on other SVG elements with ‘href’ attributes.

If the fetching algorithm results in an error or an empty response body, the reference URL is treated as an invalid reference.

If a valid response is returned, and the valid URL targets for the reference include specific element types, the user agent must continue by Processing the subresource document. Otherwise (if only entire-document the URL references are valid), then the fetched document is the referenced resource.

16.1.7.3. Processing the subresource document

Otherwise, the subresource must be parsed to identify the target element. If the fetched document is a type that the user agent can parse to create a document object model, it must process it in secure static mode (meaning, do not fetch any additional external resources and do not run scripts or play animations). The document model generated for an external subresource reference must be immutable (read-only) and cannot be modified.

If a document object model can be generated from the fetched file, processing the URL must continue as indicated in Identifying the target element with the parsed subresource document as the referenced document. The user agent may commence the target-identification process prior to completely parsing the document.

User agents may maintain a list of external resource URLs and their associated parsed documents, and may re-use the documents for subsequent references, so long as doing so does not violate the processing mode, caching, and CORS requirements on the resource.

16.1.7.4. Identifying the target element

For URL references to a specific element, whether the reference is valid depends on whether the element can be located within the referenced document and whether it is of an allowed type.

Using the referenced document identified in previous processing steps (either an external subresource document or the current document), the target element is identified as follows:

The target element provides the referenced resource if (and only if) it is a valid URL target for the reference.

16.1.7.5. Valid URL targets

The valid target element types for ‘href’ (or ‘xlink:href’) attributes are based on the element that has the attribute, as follows:

The valid target element types for style properties defined in this specification are as follows:

For references that allow either a reference to a target element, or to an image file (such as the mask property), the user agent must identify the target element and determine whether it is a valid target. If the resolved target element is not an allowed element type, the referenced resource is the entire document file; the target fragment is used in processing that file as with any other image.

In all other cases, if the resolved target element type (or document type) is not allowed for the URL reference, it is an invalid reference.

SVG provides an ‘a’ element, to indicate links (also known as hyperlinks or Web links). An ‘a’ element forms a link if it has a ‘href’ or ‘xlink:href’ attribute; without these attributes the ‘a’ element is an inactive placeholder for a link.

SVG 1.1 defined links in terms of the XLink specification ([XLink]), using attributes defined in the XLink namespace. SVG 2 uses an alternative set of attributes in the default namespace that are consistent with HTML links, and deprecates the XLink attributes.

The ‘a’ element may contain any element that its parent may contain, except for another ‘a’ element; the same element is used for both graphical and textual linked content. Links may not be nested; if an ‘a’ element is a descendent of another hyperlink element (whether in the SVG namespace or another namespace), user agents must ignore its href attribute and treat it as inactive. The invalid ‘a’ element must still be rendered as a generic container element.

The rendering of invalid nested links is at risk, and will likely be synchronized with any decisions regarding the rendering of unknown elements.

For pointer events processing, a linked hit region is defined for each separate rendered element contained within the ‘a’ element (according to the value of their pointer-events property), rather than for the bounding box of the ‘a’ element itself. User agents must also ensure that all links are focusable and can be activated by keyboard commands.

The remote resource (the destination for the link) is defined by a URL specified by the ‘href’ attribute on the ‘a’ element. The remote resource may be any Web resource (e.g., an image, a video clip, a sound bite, a program, another SVG document, an HTML document, an element within the current document, an element within a different document, etc.). In response to user activation of a link (by clicking with the mouse, through keyboard input, voice commands, etc.), user agents should attempt to fetch the specified resource document and either display it or make it available as a downloaded file.

Example link01 assigns a link to an ellipse.

<?xml version="1.0" standalone="no"?>
<svg width="5cm" height="3cm" viewBox="0 0 5 3" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example link01 - a link on an ellipse
  </desc>
  <rect x=".01" y=".01" width="4.98" height="2.98" 
        fill="none" stroke="blue"  stroke-width=".03"/>
  <a href="https://www.w3.org">
    <ellipse cx="2.5" cy="1.5" rx="2" ry="1"
             fill="red" />
  </a>
</svg>
Example link01 — a link on an ellipse

Example link01

View this example as SVG (SVG-enabled browsers only)

If the above SVG file is viewed by a user agent that supports both SVG and HTML, then clicking on the ellipse will cause the current window or frame to be replaced by the W3C home page.

‘a’
Categories:
Container element, renderable element
Content model:
Descriptive content, plus any element or text allowed by its parent’s content model, except for another a element. If the parent is a switch element, use the content model of the nearest ancestor that isn’t a switch.
Attributes:
DOM Interfaces:

Attribute definitions:

Name Value Initial value Animatable
href URL [URL] (none) yes
The location of the referenced object, expressed as an URL reference. Refer to the common handling defined for URL reference attributes.
Name Value Initial value Animatable
target _self | _parent | _top | _blank | <XML-Name> _self yes

This attribute should be used when there are multiple possible targets for the ending resource, such as when the parent document is embedded within an HTML or XHTML document, or is viewed with a tabbed browser. This attribute specifies the name of the browsing context (e.g., a browser tab or an (X)HTML iframe or object element) into which a document is to be opened when the link is activated:

_self
The current SVG image is replaced by the linked content in the same browsing context as the current SVG image.
_parent
The immediate parent browsing context of the SVG image is replaced by the linked content, if it exists and can be securely accessed from this document.
_top
The content of the full active window or tab is replaced by the linked content, if it exists and can be securely accessed from this document
_blank
A new un-named window or tab is requested for the display of the linked content, if this document can securely do so. If the user agent does not support multiple windows/tabs, the result is the same as _top.
<XML-Name>
Specifies the name of the browsing context (tab, inline frame, object, etc.) for display of the linked content. If a context with this name already exists, and can be securely accessed from this document, it is re-used, replacing the existing content. If it does not exist, it is created (the same as '_blank', except that it now has a name). The name must be a valid XML Name [XML11], and should not start with an underscore (U+005F LOW LINE character), to meet the requirements of a valid browsing context name from HTML.

The normative definitions for browsing contexts and security restrictions on navigation actions between browsing contexts is HTML [HTML], specifically the chapter on loading web pages.

Previous versions of SVG defined the special target value '_replace'. It was never well implemented, and the distinction between '_replace' and '_self' has been made redundant by changes in the HTML definition of browsing contexts. Use '_self' to replace the current SVG document.

The value '_new' is not a legal value for target. Use '_blank' to open a document in a new tab/window.

Name Value Initial value Animatable
download any value (if non-empty, value represents a suggested file name) (none) no
ping space-separated valid non-empty URL tokens [HTML] (none) no
rel space-separated keyword tokens [HTML] (none) no
hreflang A BCP 47 language tag string [HTML] (none) no
type A MIME type string [HTML] (none) no
referrerpolicy A referrer policy string [REFERRERPOLICY] (none) no
These attributes further describe the targetted resource and its relationship to the current document. Allowed values and meaning are as defined for the a element in HTML.

The SVG Working Group is waiting on the HTML specification to clarify the processing model and algorithms for these attributes. The attributes should follow the definitions and algorithms as specified in the HTML specification.

For more details on the ongoing clarification effort, see the

HTML specification issue on processing model and algorithms for the HTML A element.

16.3. Linking into SVG content: URL fragments and SVG views

Because SVG content often represents a picture or drawing of something, a common need is to link into a particular view of the document, where a view indicates the initial transformations so as to present a closeup of a particular section of the document.

16.3.1. SVG fragment identifiers

SVG 2 Requirement: Merge the SVG 1.1 SE text and the SVG Tiny 1.2 text on fragment identifiers link traversal and add media fragments.
Resolution: SVG 2 will have media fragment identifiers.
Purpose: To align with Media Fragments URI.
Owner: Cyril (ACTION-3442)

To link into a particular view of an SVG document, the URL reference with fragment identifier needs to be a correctly formed SVG fragment identifier. An SVG fragment identifier defines the meaning of the "selector" or "fragment identifier" portion of URLs that locate resources of MIME media type "image/svg+xml".

An SVG fragment identifier can come in the following forms:

16.3.2. SVG fragment identifiers definitions

An SVG fragment identifier is defined as follows:

SVGFragmentIdentifier ::= BareName *( "&" timesegment ) |
                          SVGViewSpec *( "&" timesegment ) |
                          spacesegment *( "&" timesegment ) |
                          timesegment *( "&" spacesegment )

BareName ::= XML_Name
SVGViewSpec ::= 'svgView(' SVGViewAttributes ')'
SVGViewAttributes ::= SVGViewAttribute |
                      SVGViewAttribute ';' SVGViewAttributes

SVGViewAttribute ::= viewBoxSpec |
                     preserveAspectRatioSpec |
                     transformSpec
viewBoxSpec ::= 'viewBox(' ViewBoxParams ')'
preserveAspectRatioSpec = 'preserveAspectRatio(' AspectParams ')'
transformSpec ::= 'transform(' TransformParams ')'

where:

SVG view box parameters are applied in order, as defined in CSS Transforms specification (e.g. SVG view is transformed as defined in ViewBoxParams, then as defined in TransformParams).

Spaces are allowed in fragment specifications. Commas are used to separate numeric values within an SVG view specification (e.g., #svgView(viewBox(0,0,200,200))) and semicolons are used to separate attributes (e.g., #svgView(viewBox(0,0,200,200);preserveAspectRatio(none))).

Fragment identifiers may be url-escaped according to the rules defined in CSS Object Model (CSSOM) specification. For example semicolons can be escaped as %3B to allow animating a (semi-colon separated) list of URLs because otherwise the semicolon would be interpreted as a list separator.

The four types of SVGViewAttribute may occur in any order, but each type may only occur at most one time in a correctly formed SVGViewSpec.

When a source document performs a link into an SVG document, for example via an HTML anchor element ([HTML]; i.e., <a href=...> element in HTML) or an XLink specification [xlink], then the SVG fragment identifier specifies the initial view into the SVG document, as follows:

16.3.3. Predefined views: the ‘view’ element

The ‘view’ element is defined as follows:

‘view’
Categories:
None
Content model:
Any number of the following elements, in any order: script, style
Attributes:
DOM Interfaces:

We have resolved to remove viewTarget attribute.

Resolution: Paris 2015 F2F Day 3.

Owner: BogdanBrinza.

16.4. DOM interfaces

16.4.1. Interface SVGAElement

An SVGElement object represents an ‘a’ element in the DOM.

[Exposed=Window]
interface SVGAElement : SVGGraphicsElement {
  [Reflect, SameObject] readonly attribute SVGAnimatedString target;
  [Reflect] attribute DOMString download;
  [Reflect] attribute USVString ping;
  [Reflect] attribute DOMString rel;
  [Reflect="rel", SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
  [Reflect] attribute DOMString hreflang;
  [Reflect] attribute DOMString type;

  attribute DOMString referrerPolicy;
};

SVGAElement includes SVGURIReference;
SVGAElement includes HyperlinkElementUtils;

The set the url algorithm for an SVGAElement:

  1. Set this element’s url to null.
  2. Let url be failure.
  3. If this element’s ‘href’ content attribute is present:
    1. Set url to the result of encoding-parsing a URL given this element’s ‘href’ content attribute’s value, relative to this element’s node document.
  4. Otherwise, if this element’s ‘xlink:href’ content attribute is present:
    1. Set url to the result of encoding-parsing a URL given this element’s ‘xlink:href’ content attribute’s value, relative to this element’s node document.
  5. If url is not failure, then set this element’s url to url.

To update href for an SVGAElement:

  1. If this element’s ‘xlink:href’ content attribute is present:
    1. Set the element’s ‘xlink:href’ content attribute’s value to the element’s url, serialized.
  2. Otherwise, set the element’s ‘href’ content attribute’s value to the element’s url, serialized.

The referrerPolicy IDL attribute reflects the ‘referrerpolicy’ content attribute, limited to only known values.

16.4.2. Interface SVGViewElement

An SVGViewElement object represents a ‘view’ element in the DOM.

[Exposed=Window]
interface SVGViewElement : SVGElement {};

SVGViewElement includes SVGFitToViewBox;